You might also be able to rent from a range locally. Don't know if they are legally required to have strict over-21 policies, but if they do, just grab a relative and have fun shooting everything they let you.

And if you ever make it down to Kerrville, I'll take you out. Hopefully someone closer will offer the same.

You can still legally possess a handgun at 18. You will need to be gifted the gun and your parents will need to buy your ammo for you. Fossil Pointe near Decatur might let you shoot. The guy running the pistol range also does training.